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Abstract

  1. Top of page
  2. Abstract
  3. Homeostatic Control of the Internal Milieu
  4. Disruption of Homeostasis: Insulin and Diabetes, Hepcidin and Hemochromatosis
  5. Defining and Classifying Hemochromatosis
  6. Defining and Classifying Disorders due to Tissue Iron Accumulation
  7. Clinical Aspects
  8. Conclusions and Perspectives
  9. References

This review acknowledges the recent and dramatic advancement in the field of hemochromatosis and highlights the surprising analogies with a prototypic endocrine disease, diabetes. The term hemochromatosis should refer to a unique clinicopathologic subset of iron-overload syndromes that currently includes the disorder related to the C282Y homozygote mutation of the hemochromatosis protein HFE (by far the most common form of hemochromatosis) and the rare disorders more recently attributed to the loss of transferrin receptor 2, HAMP (hepcidin antimicrobial peptide), or hemojuvelin or to certain ferroportin mutations. The defining characteristic of this subset is failure to prevent unneeded iron from entering the circulatory pool as a result of genetic changes compromising the synthesis or activity of hepcidin, the iron hormone. Like diabetes, hemochromatosis results from the complex, nonlinear interaction between genetic and acquired factors. Depending on the underlying mutation, the coinheritance of modifier genes, the presence of nongenetic hepcidin inhibitors, and other host-related factors, the clinical manifestation may vary from simple biochemical abnormalities to severe multiorgan disease. The recognition of the endocrine nature of hemochromatosis suggests intriguing possibilities for new and more effective approaches to diagnosis and treatment. (HEPATOLOGY 2007.)

Few areas of medicine have witnessed discoveries as momentous as those recently made in the field of iron metabolism. Since 1996, the major proteins involved in iron transport or the systemic control of iron homeostasis have been identified, and genetic causes have been found for most forms of tissue iron overload. Hemochromatosis (HC) is no exception. Over the past decade, our definition of this disorder has been changing, evolving, stretching, and twisting to accommodate an increasingly rich and rapid succession of discoveries, particularly those that have emerged from genetic research. This review re-examines certain time-honored concepts of hemochromatosis in light of the recent, dramatic achievements in the field and offers a view of the disease based on the current understanding of its pathophysiology, which also has implications for its diagnosis and management.

Homeostatic Control of the Internal Milieu

  1. Top of page
  2. Abstract
  3. Homeostatic Control of the Internal Milieu
  4. Disruption of Homeostasis: Insulin and Diabetes, Hepcidin and Hemochromatosis
  5. Defining and Classifying Hemochromatosis
  6. Defining and Classifying Disorders due to Tissue Iron Accumulation
  7. Clinical Aspects
  8. Conclusions and Perspectives
  9. References

The survival of living organisms depends on their ability to maintain homeostasis, that indispensable level of “constancy in the internal milieu” conceptualized by Claude Bernard in the mid-nineteenth century. However, the organism's inevitable interaction with the environment can provoke dramatic changes in the chemical and physical characteristics of the internal milieu, and these must be corrected if disease is to be averted. In mammals, the endocrine system is one of the body's main tools for maintaining internal variables (body temperature, pH, water and ion balance, and a host of others) within the limits compatible with life. It restores homeostasis through various mechanisms. One of the best known is probably negative feedback, the mechanism by which glucose metabolism is regulated (Fig. 1A). This monosaccharide is a major source of energy; too little may lead to starvation, but too much is toxic. Unneeded fluctuations in blood glucose levels are therefore rapidly and efficiently controlled by the negative-feedback machinery constructed around the pancreatic hormone insulin (Fig. 1A,B).

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Figure 1. Negative-feedback systems for the control of glucose and iron homeostasis and their breakdown. (A) Essential components of feedback systems for the maintenance of homeostasis in glucose and iron metabolism. Each component plays a specific role in the process by which an organism regulates its own internal environment. A sensor detects potentially dangerous changes in the internal environment and reports them to a control center [bone morphogenic protein receptor (BMPr)]. The latter responds by activating an effector protein, whose function is to restore homeostasis by interacting with a specific target/receptor. (B,C) Homeostatic regulation of glucose and iron metabolism. Excess glucose in the bloodstream is detected by specialized sensors in pancreatic beta cells, and the control center responds to this threat by releasing insulin. This effector peptide diffuses through the body, interacting with specific receptors on muscle cells, adipocytes, and hepatocytes and instructing them to remove glucose from the blood and use it for energy production or store it for later use as glycogen. As a result of this activity, nontoxic blood-glucose concentrations are restored. In a similar manner, excess circulatory iron is detected by a sensor system in the hepatocytes, which is composed of proteins involved in iron uptake and/or hepcidin transcription (HFE, TfR2, and HJV). The liver releases hepcidin, which interacts with the iron-exporter ferroportin expressed on the surfaces of macrophages and enterocytes. The result is the degradation of ferroportin, decreased iron release from macrophages and intestinal cells, and ultimately the restoration of safe blood levels of iron. (D,E) Disruption of homeostasis: diabetes and hemochromatosis. (D) Defective insulin production (caused by immune-mediated or secondary destruction of pancreatic beta cells or genetic defects that impair glucose sensing or insulin synthesis) or reduced insulin sensitivity can cause unchecked increases in blood glucose levels and diabetes. (E) Similarly, defective hepcidin synthesis (caused by a massive loss of hepatocytes or genetic and acquired factors that impair iron sensing or hepcidin synthesis) or reduced hepcidin sensitivity can lead to progressive increases in serum iron levels and hemochromatosis.

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Iron metabolism is regulated by surprisingly similar mechanisms (Fig. 1A,C). Like glucose, iron is an essential nutrient. It is required for hemoglobin synthesis and vital enzyme activities, but excess iron promotes noxious free-radical reactions. The control center that keeps blood levels of iron within the narrow physiological range is the hepatocyte. Here, specialized sensors respond to excess iron by stimulating the synthesis and release of the effector hormone, hepcidin. This defensin-like peptide, encoded by the hepcidin antimicrobial peptide (HAMP) gene, diffuses through the body, interacting with the iron-exporter ferroportin expressed on the surfaces of iron-rich macrophages and intestinal cells. As a result of this interaction, ferroportin is internalized and degraded,1 and the unneeded iron remains in the cell, in which it is saved for future use in the form of ferritin, just as excess glucose is stored as glycogen (Fig. 1C). The diminished release of iron restores blood levels to the nontoxic range, thus removing the stimulus for further hepcidin synthesis, and ferroportin gradually resumes its iron-exporting activity. This mechanism ensures the maintenance of circulating iron levels that can meet the body's erythropoietic needs without posing an oxidative threat to the cells.

Iron is also essential for a number of bacterial and viral pathogens, and hepcidin also plays a key role in depriving pathogens of a required micronutrient. In this case, hepcidin is released in response to inflammatory signals and cytokines. Its suppression of ferroportin activity prevents iron from entering the bloodstream, in which it could be used by proliferating pathogens. Later, when iron is needed in the bone marrow for erythropoiesis (for example, during hypoxia or anemia), hepcidin expression ceases,2 and ferroportin is once again available to transfer iron to the circulation from enterocytes and macrophages.

Although our understanding of glucose sensing is quite extensive, we are just beginning to dissect the mechanisms underlying hepatocyte iron sensing (Fig. 1A). HFE and transferrin receptor 2 (TfR2), which are thought to be involved in transferrin iron uptake, might play a role in conveying iron signals to the hepatocyte control center, but the details of this process are still unclear. Both are clearly important for hepcidin expression: the loss of either causes hepcidin insufficiency and iron overload in rodents and humans (discussed later). However, the most important hepcidin regulator is hemojuvelin (HJV),3 a coreceptor for bone morphogenic protein (BMP) ligands that is essential for HAMP transcription in hepatocytes.4, 5

Disruption of Homeostasis: Insulin and Diabetes, Hepcidin and Hemochromatosis

  1. Top of page
  2. Abstract
  3. Homeostatic Control of the Internal Milieu
  4. Disruption of Homeostasis: Insulin and Diabetes, Hepcidin and Hemochromatosis
  5. Defining and Classifying Hemochromatosis
  6. Defining and Classifying Disorders due to Tissue Iron Accumulation
  7. Clinical Aspects
  8. Conclusions and Perspectives
  9. References

Similarities between the systems that control glucose and iron metabolism persist when these systems break down. Failure to maintain physiological blood levels of either nutrient can have various causes, disruption can occur at different points within the negative-feedback loop, and the consequences range from mild to life-threatening. Hormone secretory failure is a major component of both diabetes and hemochromatosis (Fig. 1D,E). Recent research has highlighted the genetic heterogeneity of hemochromatosis, but all forms identified thus far, regardless of the mutations that cause them, are due to insufficient release of hepcidin.6 The result is unrestricted iron production from macrophages and intestinal cells, which produces circulatory and later tissue iron overload (Fig. 1E). Hepcidin production, like that of insulin, can be impaired by genetic and acquired factors. Hepcidin deficiency has been associated with the loss of hepcidin itself, HJV, HFE, or TfR2 in humans3,7-9 and with the inactivation of CCAAT/enhancer binding protein α and mothers against decapentaplegic homolog 4 (Smad4) in mice.10, 11 As for the contribution of nongenetic factors, ethanol abuse (long associated with iron overload) is now believed to inhibit hepcidin transcription.12, 13 Similar effects can be produced by toxic and viral insults (for example, the hepatitis C virus may inhibit hepcidin expression14). The unexplained hepatic iron overload associated with acute liver failure and chronic end-stage liver disease, which closely resembles that of hemochromatosis,15 may be a manifestation of diminished hepcidin output due to a markedly reduced functional hepatic mass. Finally, the rapidly fatal iron-loading disease known as neonatal hemochromatosis is now felt to be caused by the immune-mediated destruction of another major hepatic iron protein,16 which is currently unidentified. It might be linked to hepcidin. In short, not only genetic but also acquired factors seem to be able to impair liver hepcidin output and therefore cause plasma and tissue iron overload. Therefore, all these acquired factors may act as potential modifiers of the (genetically determined) hemochromatosis phenotype.

Insulin resistance is another well-known component of diabetes, and by analogy, one would expect to see a hemochromatosis phenotype in the presence of factors rendering ferroportin insensitive to hepcidin-induced degradation (Fig. 1E). Indeed, discrete mutations in ferroportin that reportedly cause hepcidin resistance in vitro are believed to be the cause of certain rare forms of ferroportin disease that resemble HFE hemochromatosis.17 It can be easily predicted that other genetic and environmental modifiers of hepcidin sensitivity able to interfere with the hepcidin-ferroportin interaction and cause circulatory iron overload will be soon identified.

Defining and Classifying Hemochromatosis

  1. Top of page
  2. Abstract
  3. Homeostatic Control of the Internal Milieu
  4. Disruption of Homeostasis: Insulin and Diabetes, Hepcidin and Hemochromatosis
  5. Defining and Classifying Hemochromatosis
  6. Defining and Classifying Disorders due to Tissue Iron Accumulation
  7. Clinical Aspects
  8. Conclusions and Perspectives
  9. References

The term hemochromatosis has been inconsistently used in the literature (and in clinical practice). Sometimes, it is used to vaguely refer to tissue iron overload, to tissue iron overload with organ damage, or to genetically determined iron overload or, more recently, to indicate HFE-related iron overload. I suggest that the term hemochromatosis should be used to refer to a unique clinicopathologic subset of iron-overload syndromes that currently includes the disorder related to HFE C282Y homozygosity (the prototype and by far the most common form of hemochromatosis) and the rare disorders more recently attributed to the loss of TfR2, HAMP, or HJV or to certain ferroportin mutations. The defining characteristic of this subset is failure to prevent unneeded iron from entering the circulatory pool as a result of genetic changes compromising the synthesis or activity of hepcidin. All forms of hemochromatosis share the following basic features (Table 1):

  • Hereditary disease. HC can be associated with mutations of various genes. The genetic forms recognized thus far (Table 1), except those associated with ferroportin mutations, are transmitted as autosomal recessive traits. However, the list of hemochromatosis genes is destined to grow as ongoing research uncovers other pathogenic mutations of proteins involved in hepcidin transcription, processing, secretion, stability, hepcidin-ferroportin interaction, or ferroportin degradation.

  • Increased transferrin saturation. The first biochemical manifestation of hemochromatosis is increased saturation of the iron transporter, transferrin. This is accompanied by increased non–transferrin-bound iron, particularly in the portal blood. All this reflects the uncontrolled release of iron into the bloodstream by enterocytes and macrophages, a process normally down-regulated by hepcidin (Fig. 2). Hepatic hepcidin expression is significantly impaired in HFE, TfR2, and HJV knockout mice,18–20 and the iron-overload syndromes associated with HFE, TfR2, HAMP, and HJV mutations are all characterized by inadequate hepcidin synthesis.3, 7–9

  • Iron overload subsequently involving parenchymal cells. Apart from menstruation, the body has no effective means of significantly reducing plasma iron levels. Therefore, without therapeutic intervention, overloads in this compartment will lead to the progressive accumulation of iron in the parenchymal cells of key organs, creating a distinct risk for tissue iron overload. The deposits are particularly evident in hepatocytes (Fig. 3) but may also be found in parenchymal cells of the endocrine glands and heart. This stage is reflected in increasing serum ferritin (SF) levels. The time of onset and pattern of organ involvement vary, depending on the rate and magnitude of the plasma iron overloading, which depend, in turn, on the underlying mutation. For this reason, the milder adult-onset syndromes (HFE-related and TfR2-related) are often distinguished from the more severe juvenile-onset forms (HJV-related and HAMP-related),6 but the implied dichotomy can be misleading. These forms are merely 2 points on a phenotypic continuum representing the same underlying syndrome.

  • Unimpaired erythropoiesis. Iron is abundant (even excessive) in the plasma, and its transport to the bone marrow by transferrin and incorporation into hemoglobin are unaffected by the genetic defects that cause hemochromatosis. Consequently, anemia is not a feature of this iron-overload syndrome, and therapeutic phlebotomy is tolerated quite well.

  • Defective hepcidin synthesis or activity. As previously discussed, any genetic defect that negatively affects the synthesis or secretion of hepcidin or its ability to down-regulate ferroportin activity can lead to hemochromatosis (Fig. 1E). It is thus the central pathogenic factor for all forms of hemochromatosis6 (Fig. 2). There is highly compelling evidence that HJV, via BMP-SMAD4 signaling, is required for hepcidin transcription,4 but the roles played by HFE and TfR2 are still unclear21–23 (Fig. 4). If hemochromatosis is defined by the presence of all the previously discussed features (Table 1), other iron-overload syndromes can be excluded from this subset if they lack at least one of its defining characteristics. The clearly hereditary disorders listed in Table 2 include several that do not qualify as hemochromatosis because they fail (first of all) to satisfy the inclusion criterion of unimpaired erythropoiesis. In aceruloplasminemia, for example, reduced iron export from cells decreases the iron loading of serum transferrin (Fig. 2), creating a distinct risk for anemia (Table 3). The same is true of the disease listed in the Online Mendelian Inheritance in Man database as type 4 hemochromatosis.24 Despite its undeniable imperfections, I prefer the term ferroportin disease25 as a simpler and less misleading name for this disorder, which is classically caused by ferroportin mutations that reduce macrophage iron export. One of its intrinsic features is an impaired response to increased bone marrow requirements for iron (for example, those provoked by heavy menstruation or aggressive phlebotomy), which obviously constitutes a risk for anemia.25 This classic form of ferroportin disease also differs from hemochromatosis in other fundamental respects, including the absence of the early expansion of the circulatory iron pool. Indeed, transferrin saturation (TS) rises only in the later stages of this disease. It is also associated with massive iron overloading of the reticuloendothelial macrophages, which typically present an iron-deprivation phenotype in hemochromatosis (Figs. 3 and 5). Abdominal magnetic resonance imaging (MRI) is a very useful, noninvasive tool for identifying this macrophage-iron signature of classic ferroportin disease and differentiating this disorder from hemochromatosis26 (Fig. 5).

Table 1. Hemochromatosis in a Nutshell
DefinitionIron-overload disease caused by a genetically determined failure to prevent unneeded iron from entering the circulatory pool
Distinguishing features1. Hereditary (usually autosomal recessive) trait
 2. Early and progressive expansion of the plasma iron compartment
 3. Progressive parenchymal iron deposition that can cause severe damage and disease involving the liver, endocrine glands, heart,   and joints
 4. Nonimpaired erythropoiesis and optimal response to therapeutic phlebotomy
 5. Defective hepcidin synthesis or activity
Postulated pathogenic basisGene mutations leading to inappropriately low hepatic synthesis or impaired peripheral activity of hepcidin
Recognized genetic causesPathogenic mutations of HFE, TfR2, HJV, or HAMP and certain ferroportin mutations
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Figure 2. The main pathogenic pathways of human hereditary iron overload disorders. The circulatory iron pool is essential for supplying iron to the erythropoietic organs and other functional sites. Entry into the bloodstream of iron from the intestine and storage sites depends largely on the presence of ferroportin and the auxiliary circulatory or cell-associated ferroxidases (such as ceruloplasmin or hephestin, respectively), which help to load iron onto the transporter transferrin. The half-life and membrane activity of ferroportin are mainly dependent on the circulating levels of hepcidin. There are 2 hereditary disorders in which iron accumulates in tissues because of impaired iron export activity: ferroportin disease, in which the underlying defect is the functional loss of the specialized iron carrier ferroportin, and aceruloplasminemia, which is caused by the loss of a fundamental ferroxidase. Compared with the tissue iron compartment, the circulatory iron pool may be disproportionately low, and this may pose a risk for iron-restricted erythropoiesis and anemia. In atransferrinemia, iron absorbed from the intestine or released from macrophages cannot be transported to the bone marrow because of the absence of transferrin. Anemia is constant and severe. Tissue overload generally occurs as the unphysiologically high levels of non–transferrin-bound forms of iron pass into the cells. In the case of hemochromatosis, ferroportin-mediated iron export goes unchecked because of the loss of hepcidin or resistance to its effects. The result is circulatory iron overload followed by tissue overload as the excess iron passes from the bloodstream into parenchymal cells. Here, the ratio of circulatory iron to tissue iron is high.

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Figure 3. Iron accumulation patterns in hemochromatosis and ferroportin disease (Perls' Prussian blue stain). (A) HFE hemochromatosis, male, age 36, LIC of 270 μmol/g of dry weight: parenchymal cell iron deposits present a typical portocentral gradient. (B) TfR2 hemochromatosis, male, age 24, LIC of 62 μmol/g of dry weight: iron deposits are localized in parenchymal cells, mainly in the periportal areas, as they are in HFE hemochromatosis. (C) HJV-associated hemochromatosis, female, age 40, LIC of 514 μmol/g of dry weight: massive iron overload of hepatocytes with a panlobular distribution. The arrow shows an iron-free focus. (D) Patient with ferroportin disease at age 59, LIC of 646 μmol/g of dry weight: severe hepatic iron overload, largely in Kupffer cell aggregates and, to a lesser extent, hepatocytes. (E) Liver specimen from the same patient at age 79, after 20 years of phlebotomy, LIC of 55 μmol/g of dry weight: iron deposits are reduced but still evident in isolated or coalescent Kupffer cells (arrows). (F) Early-stage ferroportin disease in a female, age 21, LIC of 110 μmol/g of dry weight. Note the early accumulation of iron in the Kupffer cells (arrows).

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Figure 4. Molecular basis for hereditary hemochromatosis. The transcription of the hepcidin gene, HAMP, requires the interaction of membrane-bound HJV, a coreceptor for BMP, with bone morphogenic protein receptor (BMP-R) I and BMP-R II,4 and it is probably down-regulated by endogenous BMP ligands and agonists (currently unidentified) and by soluble hemojuvelin (sHJV) produced in muscle and other peripheral tissues.5 The influx of iron from the bloodstream to the hepatocyte might be sensed directly by the HJV/BMP/BMP-R signaling complex, but it seems more likely that TfR1 and TfR2 contribute to this process by mediating the uptake of transferrin-bound (Tf) and non–transferrin-bound iron (Fe). Normal HFE is clearly associated with TfR1 (as depicted in the figure), and some investigators maintain that it also interacts with TfR2.21 Animal and human studies indicate that HFE and TfR2 are both important for hepcidin expression in hepatocytes but less so than HJV. Their precise roles are still unclear. One group has provided in vitro evidence that TfR2, HFE, and TfR1 constitute a functional sensing unit responsible for conveying the iron signal to hepcidin.21 They now maintain that HFE and TFR2 serve to amplify BMP signaling through an HJV/BMP receptor pathway22 (pathway A). Another possibility is that HFE(-TfR1) and TfR2 are both components of a signaling pathway that is independent of HJV-BMP (pathway B). This view is consistent with the recent finding of preserved BMP signaling in hepatocytes from HFE and TfR2 knockout mice.23 Finally, HFE(-TfR1) and TfR2 could be components of distinct but complementary pathways that coregulate hepcidin expression (pathway C). This hypothesis is supported by the fact that combined pathogenic mutations of HFE and TfR2 are associated with a more severe iron-overload phenotype than those caused by the mutation of HFE or TfR2 alone.9

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Table 2. Proposed Classification of Disorders Associated with Iron Accumulation
  • *

    Although severe transfusional iron overload develops during the treatment of these anemias, iron deposits in the hepatic parenchyma are already present before transfusions are started; this may be caused by increased iron absorption and recycling in response to inefficient erythropoiesis caused by secondary hepcidin down-regulation.

  • Massive hepatic iron loading and generally fatal perinatal liver failure. The hereditary nature of this disorder is uncertain. It has been recently attributed to an alloimmune phenomenon.

  • Particularly frequent among Africans who drink a traditional beer brewed in nongalvanized steel drums, this disorder was once attributed exclusively to dietary excess. Segregation analysis has led to the conclusion that an unidentified iron-loading gene may confer susceptibility to the disease. Ferroportin has been implicated in this disorder as a possible modifier gene.

Hereditary
 Hemochromatosis
 Ferroportin disease
 Aceruloplasminemia
 A(hypo)transferrinemia
 Friedreich's ataxia
 Hereditary iron-loading anemias (thalassemia, hereditary sideroblastic anemia, and chronic hemolytic anemia)*
Acquired
 Dietary
 Parenteral and transfusional
 Anemia of inflammation
 Acquired iron-loading (hemolytic and sideroblastic) anemias
 Long-term hemodialysis
 Chronic liver disease (alcoholic, viral, and dysmetabolic)
 Porphyria cutanea tarda
 Alloimmune neonatal hemochromatosis
 Iron overload in sub-Saharan Africa
Table 3. Main Features of Hemochromatosis and Other Hereditary Iron Overload Disorders in Humans.
DisorderAffected Gene (symbol/location)Known or Postulated Gene Product Function*EpidemiologyGeneticsMechanism for Cellular Iron AccumulationClinical Onset (Decade)Main Clinical ManifestationClinical Course
  • *

    As reported in www.genetest.org.

  • In several clinically reported cases, molecular characterization has not been performed.

I. HemochromatosisHemochromatosis gene (HFE/ 6p21.3)• Interaction with transferrin receptor 1 • Uptake of transferrin-iron • Hepcidin regulator• Affects Caucasians of northern European descent • Associated genetic defect (C282Y homozygosity) highly prevalent: 1/200-300Autosomal recessiveIncreased iron influx3°-5°Liver DiseaseMild-Severe
 Transferrin-receptor 2 (TfR2/ 7q22)Uptake of transferrin and non transferrin-bound ironHepcidin regulator12 published mutations in 32 patients from 13 pedigrees     
 Hepcidin antimicrobial peptide (HAMP/19q13.1)Down-regulation of iron efflux from macrophages, enterocytes, placenta, through degradation of ferroportin5 published mutations in 10 patients from 7 pedigrees  2°-3°Hypogonadism and cardiac diseaseSevere
 Hemojuvelin (HJV/ 1p21)Co-receptor for bone morphogenic proteinsHepcidin transcriptional regulator33 published mutations in 70 patients from 59 pedigreesMost prevalent mutation: G230V     
 Solute carrier family 40 (iron-regulated transporter), member 1 (SLC40A1/ 2q32)Iron export from cells including macrophages, enterocytes, placental cells3 published mutations in 20 patients from 3 pedigreesAutosomal dominant 3°-5°Liver diseaseUnclear
II. Ferroportin DiseaseSolute carrier family 40 (iron-regulated transporter), member (1SLC40A1/ 2q32)Iron export from cells including macrophages, enterocytes, placental cells21 published mutations in 112 patients from 31 pedigreesMost prevalent mutations: Val162del and A77DAutosomal dominantDecreased iron efflux4°-5°Liver abnormalitiesMarginal anemiaMild
III. AceruloplasminemiaCeruloplasmin (CP/ 3q23-q25)Iron efflux from cells37 published mutations in 45 pedigrees*Autosomal recessiveDecreased iron efflux2°-3°• Neurologic manifestationsAnemiaSevere
IV. A(hypo)transferrinemiaTransferrin (Tf/ 3q21)Iron transport in the bloodstream4 mutations in 10 patients from 8 pedigreesAutosomal recessiveIncreased iron influx1°-2°AnemiaSevere
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Figure 5. MRI scans from patients with hemochromatosis and other iron-loading disorders. (A,B) A 50-year-old male with HFE hemochromatosis before and after phlebotomy, respectively. Before the treatment (SF, 1120 ng/mL; LIC, 350 μmol/g of dry weight), MRI reveals a massive iron overload in the liver with an iron-poor spleen. The hepatic signal intensity is reduced after iron removal (SF, 65 ng/mL; LIC, 35 μmol/g of dry weight). (C) A 49-year-old female with ferroportin disease (SF, 1789 ng/mL; LIC, 240 μmol/g of dry weight). Massive iron deposits are evident in the liver and in the spleen and spine. (D) A 53-year-old male with ferroportin disease after phlebotomy (SF, 90 ng/mL; LIC, 30 μmol/g of dry weight). SF has normalized, but iron accumulation is still detectable in the spleen and spine (arrow). (E) A female (age 32) with HJV-related hemochromatosis (LIC, 260 μmol/g of dry weight). The MRI pattern resembles that of HFE hemochromatosis. (F) A 35-year-old woman with B-thalassemia (LIC, 150 μmol/g of dry weight). The MRI iron signals are increased in both the liver and spleen-spine, but those of the latter region are much higher than those of the liver, reflecting a posttransfusion iron overload predominantly involving the macrophage compartment.

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In listing iron-overload states in Table 2, 1 I have intentionally avoided the terms primary and secondary, which are also used inconsistently in the literature. Is a primary disorder one due to a primary defect of iron metabolism, that is, a defect that is not the result of some other disorder? Or is it a disorder caused by the mutation of a protein that plays a primary role in iron homeostasis, that is, a role of first-rank importance? If so, what does first-rank mean? For clinicians, it may be more helpful to distinguish between inherited disorders that may require pedigree studies and genetic counseling and those that are acquired, in which case an external cause may need to be identified and, if possible, eliminated.

Defining and Classifying Disorders due to Tissue Iron Accumulation

  1. Top of page
  2. Abstract
  3. Homeostatic Control of the Internal Milieu
  4. Disruption of Homeostasis: Insulin and Diabetes, Hepcidin and Hemochromatosis
  5. Defining and Classifying Hemochromatosis
  6. Defining and Classifying Disorders due to Tissue Iron Accumulation
  7. Clinical Aspects
  8. Conclusions and Perspectives
  9. References

Although defining and classifying iron overload states by their hereditary or acquired nature is simple and effective, subcategorizing these disorders according to mechanistic aspects may even offer interesting clues for diagnosis and treatment (Fig. 6 and Table 4). From a mechanistic point of view, iron accumulation in cells essentially results from either increased cell iron influx or decreased efflux or, as we are just now beginning to recognize, altered subcellular iron traffic (Fig. 6). In humans, many pathologic states result from systemic iron overload (for example, hemochromatosis, posttransfusion siderosis, and ferroportin disease). Others are rather caused by iron misdistribution and are associated with the regional accumulation of iron in subcellular compartments (for example, mitochondria in Friedreich's ataxia) or certain cell types and organs (for example, macrophages in anemia of chronic disease and basal ganglia in neurodegenerative disorders). In strict terms, the latter disorders may not all qualify as true iron-overload states, as the total body iron content may not be increased. If we now consider the mechanisms depicted in Fig. 6, we realize that most iron-loading states in humans belong to type 1, in which hepcidin deficiency is the key pathogenic factor leading to increased plasma iron and, consequentially, higher cell iron influx (Table 4).

Table 4. Pathophysiologic Classification of Iron-Accumulation States in Humans
Type 1. Increased cellular iron influx (hepcidin is not adequately produced or nonfunctional)
Subtype A
 Mechanisms: The circulatory iron pool is expanded because of the uncontrolled release of iron from enterocytes and macrophages even after erythropoietic needs have been met.
 Anemia: Absent
 Phlebotomy: Indicated and well tolerated
 Examples: Hemochromatosis (HFE-related, TfR2-related, HJV-related, HAMP-related, or, in rare cases, ferroportin-related) and oral or parental iron overload (alcohol or hepatitis viruses?)
Subtype B
 Mechanisms: The circulatory iron pool is expanded because of the up-regulated release of iron from enterocytes and macrophages, which is stimulated by iron-restricted or inefficient erythropoiesis and/or hemolysis.
 Anemia: Present
 Phlebotomy: Usually contraindicated; iron chelators usually indicated
 Examples: A(hypo)transferrinemia and hereditary anemias associated with inefficient erythropoiesis
Type 2. Decreased cellular iron efflux (hepcidin is normally produced and functional)
 Mechanisms: The circulatory iron pool is paradoxically restricted in comparison with the amount of iron trapped in the macrophage and other cellular compartments.
 Anemia: Marginal to overt
 Phlebotomy: May or may not be effective; use with caution to avoid provoking anemia.
 Examples: Classic ferroportin disease, aceruloplasminemia, and anemia of chronic disease
Type 3. Altered intracellular or regional iron traffic (systemic iron traffic is not primarily affected)
 Mechanisms: The iron overload is due to altered iron traffic within subcellular organelles and/or in certain cell types.
 Anemia: Usually absent or marginal
 Phlebotomy: May not be effective or indicated (if anemia); iron chelators may prove effective.
 Examples: Friedreich's ataxia, hereditary sideroblastic anemias, and neurodegenerative disorders
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Figure 6. Mechanisms of cell iron accumulation. Iron accumulation in cells may result from (1) increased iron influx, (2) decreased efflux, or, more rarely, (3) altered intracellular trafficking. Examples of human diseases are reported for each category.

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Type 2 hereditary iron-overload states share important clinical features: an inappropriately low plasma iron pool (that is, low-normal TS), which poses a risk for anemia, and reduced tolerance to phlebotomy (Table 4). Prototypic examples of this category are ferroportin disease and aceruloplasminemia (see also Figs. 2 and 6). Another class of syndromes (so far underrepresented) that are associated with pathologic iron accumulation is due to disturbed subcellular iron traffic; this leads to regional iron overload and toxicity in restricted body compartments and organs (type 3 in Table 4 and Fig. 6). A classic example is Friedreich's ataxia, the most common hereditary ataxia, which is caused by a large expansion of an intronic GAA repeat, resulting in decreased expression of the target frataxin gene. The signs and symptoms of the disorder (mainly due to neurological impairment) derive from decreased expression of the protein frataxin, which chaperones iron for iron-sulfur cluster biogenesis and detoxifies iron in the mitochondrial matrix. Because of the local accumulation of iron, iron chelators seem effective in removing and relocating iron, as suggested by a recent study27 (Table 4).

Clinical Aspects

  1. Top of page
  2. Abstract
  3. Homeostatic Control of the Internal Milieu
  4. Disruption of Homeostasis: Insulin and Diabetes, Hepcidin and Hemochromatosis
  5. Defining and Classifying Hemochromatosis
  6. Defining and Classifying Disorders due to Tissue Iron Accumulation
  7. Clinical Aspects
  8. Conclusions and Perspectives
  9. References

Clinical Expressivity.

Like diabetes, hemochromatosis is a genetically heterogeneous disease that results from complex, nonlinear interactions between genetic and acquired factors. If the altered gene plays a dominant role in hepcidin synthesis (for example, HAMP itself or HJV), circulatory iron overload occurs rapidly and reaches high levels. In these cases, the modifying effects of acquired environmental and lifestyle factors will be negligible, and the clinical presentation will invariably be dramatic, with early onset (first-second decade) of full-blown organ disease, particularly heart and endocrine diseases. The heart and endocrine glands, which are more susceptible to iron toxicity, succumb to its effects earlier, and their failure will dominate the clinical picture.

In contrast, mutations involving HFE result in a genetic predisposition that requires the concurrence of host-related or environmental factors to produce disease. The clinical presentation, which usually occurs in mid life, varies from simple biochemical abnormalities to severe organ damage and disease.6 The altered HFE protein plays an essential role in this process, but its presence alone is insufficient to explain the broad spectrum of metabolic and pathologic consequences ascribed to the disease. Two main HFE mutations exist, C282Y and H63D, but only C282Y homozygosity is capable of producing clinical manifestations.6 The development of organ disease in HFE hemochromatosis obviously requires the concurrence of genetic and nongenetic factors. Coinherited mutations in other hemochromatosis genes, such as HAMP and HJV, may have a role, but they are rare.28, 29 In general, the modifier effect of iron genes has been documented in mice,30–32 and these findings have not been fully confirmed in humans.33–37 Debate continues over the roles of a fatty liver, a high body mass index,38, 39 and polymorphic changes in oxidative stress-related genes,40 but there is evidence for a strong association between alcohol and the development of hemochromatosis-related cirrhosis.41 The existence of nongenetic hepcidin inhibitors (alcohol may be one of many) raises the possibility of purely acquired forms of hemochromatosis (according to the scheme proposed in Fig. 1). It is impossible to exclude this possibility without much more complete knowledge of the nongenetic factors that can actually suppress hepcidin synthesis. However, as far as alcohol abuse is concerned, the loss of hepcidin activity that it causes would usually be episodic and transient. (The same can probably be said of other nongenetic inhibitors.) The risk for severe tissue iron overload is likely to be much greater when there is a persistent and life-long inability to up-regulate hepcidin that is genetically based. In fact, the iron overload associated with alcoholic liver disease is never as rapid or extensive as that found in fully penetrant hemochromatosis.42 In the absence of a genetically determined hepcidin deficit, alcohol and other acquired forms of hepcidin suppression are unlikely to produce per se clinically evident iron-dependent organ disease, although they may be important modifiers of an HFE hemochromatosis phenotype.

It is important to recall that, variations notwithstanding, all of these genetic mutations cause the same syndrome, the targets of iron toxicity are identical (that is, the liver, heart, endocrine glands, and joints), and the pathogenic basis of all forms is a hepcidin deficiency. Uncharacteristically, severe or anticipated-onset disease in patients with mutations of an adult-onset gene (for example, HFE) might be the result of undetected mutations in other hemochromatosis genes, such as heterozygous mutations of HAMP or HJV28, 29 or even pathogenic mutations of TfR2.9 Although the rarity of these additional genotypic abnormalities must be emphasized, the variety of genotypes that can produce a hemochromatosis phenotype highlights the importance of defining and classifying this disease as a unique clinicopathologic entity.

Diagnosing Hemochromatosis.

Similar to hyperglycemia in diabetes, the first abnormality in hemochromatosis is hyperferremia (which translates into increased TS). Ideally, measuring circulating hepcidin would help in diagnosing the disorder. However, a reliable test to assess bioactive serum hepcidin is not yet available; hepcidin measurement in the urine, widely used for research purposes in specialized centers, needs to be validated in controlled studies, and its reproducibility needs to be tested across different centers.

Today, in patients with signs and symptoms and/or organ disease suggestive of hemochromatosis, the diagnosis is based on the presence of blood and tissue iron overload and a pathogenic mutation in a hemochromatosis gene, usually C282Y HFE homozygosity. In fact, untreated C282Y homozygotes with cirrhosis, diabetes, or cardiomyopathy invariably have abnormal TS rates and SF levels. Prior to the identification of HFE, an evaluation of the hepatic iron content and distribution by liver biopsy was the method of choice for diagnosing hemochromatosis (Fig. 3). Today, the demonstration of C282Y homozygosity in a subject with a high TS rate and an elevated SF level is sufficient for diagnosis,6 although liver biopsy can still play an important role in establishing the prognosis in patients with an SF level above 1000 ng/L, increased aminotransferases, and hepatomegaly.43–45 The liver iron concentration (LIC) can be assessed noninvasively by MRI over a wide range of iron concentrations46, 47 (Fig. 5).

Symptomatic subjects with clear signs of circulatory and tissue iron overload but negative results in tests for HFE mutation may have pathogenic mutations in other HC genes. Genetic testing for non-HFE HC is complex, however, and it is not widely available. An alternative approach in these cases is based on the biopsy demonstration of a hemochromatotic pattern of hepatic iron distribution (Fig. 3), together with a hepatic iron index (LIC, in micromoles, divided by age) greater than 1.9.6 The latter finding was considered diagnostic for human leukocyte antigen–linked HC in the pre-HFE era.

Therapy.

Like diabetes, hemochromatosis should ideally be treated with hormone replacement. This is not yet possible, but noxious blood iron levels can be reduced by the ancient, but nonetheless effective, practice of bloodletting. Phlebotomy is indeed the current standard of care in HC. One unit (400-500 mL) of blood contains approximately 200-250 mg of iron. Weekly phlebotomy can restore safe blood levels of iron (reflected by SF levels of less than 20-50 μg/L and TS below 30%) within 1-2 years. Maintenance therapy, which typically involves removal of 2-4 units a year, must then be continued for life to keep TS and SF normal. Phlebotomy is generally regarded as a safe and effective means for removing iron from tissues and preventing complications, although this idea has never been validated in controlled studies, for obvious ethical reasons. The goal of bloodletting during the iron-depletion stage is generally the induction of a mildly iron-deficient state. During maintenance, therapy is usually target to keep ferritin levels below 50 ng/mL, but less aggressive regimens might actually be more beneficial. Excessive iron depletion could conceivably have negative rebound effects because anemia/hypoxia and/or iron deficiency are major suppressors of hepcidin synthesis.

Conclusions and Perspectives

  1. Top of page
  2. Abstract
  3. Homeostatic Control of the Internal Milieu
  4. Disruption of Homeostasis: Insulin and Diabetes, Hepcidin and Hemochromatosis
  5. Defining and Classifying Hemochromatosis
  6. Defining and Classifying Disorders due to Tissue Iron Accumulation
  7. Clinical Aspects
  8. Conclusions and Perspectives
  9. References

The liver and the pancreas originate from the same bipotential cell population in the primitive gut,48 both have important, well-known exocrine functions (the production of bile and digestive enzymes that facilitate food absorption), and the similarities do not end there. Few of us think of the liver as an endocrine gland, and yet it is the source of secreted peptides that are every bit as important as the more familiar hormones produced by the pancreas: above all, the iron-regulatory hormone hepcidin.

In healthy subjects, blood iron levels fluctuate with food intake, exercise, menses, and other physiologic factors, but they are kept within an acceptable range by hepcidin. This hormone helps the body keep excess iron out of the bloodstream. In subjects with hemochromatosis, blood iron levels are not adequately controlled by hepcidin. The analogies with glucose homeostasis, insulin, and diabetes are striking.

Hemochromatosis was originally considered an unusual autopsy finding that was probably alcohol-related. Over a century later, it was finally recognized as a hereditary disorder caused by a mutation of a human leukocyte antigen–linked protein thought to be involved in intestinal iron transport. Soon after the identification of HFE as the cause of the disease, other apparently unrelated hemochromatosis proteins emerged. In addition, it gradually became clear that HFE played no direct role in intestinal iron transport. The definition of hemochromatosis and its pathogenesis has thus become increasingly confusing, particularly for clinicians.

Recognizing the analogies between this disease and a classic endocrine disease such as diabetes simplifies the matter and places hereditary hemochromatosis within its natural pathogenic context. Adopting diabetes as our model, we can also foresee new and more effective approaches to the diagnosis of hemochromatosis (for example, perhaps the measurement of serum hepcidin and ferroportin-sensitivity assays based on a hepcidin clamp technique) and its treatment (for example, hormonal-peptide replacement, hepcidin synthesizers, and agonists). Like diabetes, hemochromatosis seems to be the result of mutations that were once beneficial to our ancestors. Prototypical HFE-related hemochromatosis probably arose when hunter-gatherer societies were adapting to an agriculture-based lifestyle in the iron-poor environment of northern Europe. However, the physical stress of those archaic societies has now been replaced by inactivity and an overabundance of food, and we are now witnessing the emergence of disorders, such as diabetes and hemochromatosis, related to the overefficient absorption and storage of micronutrients, such as glucose and iron.

References

  1. Top of page
  2. Abstract
  3. Homeostatic Control of the Internal Milieu
  4. Disruption of Homeostasis: Insulin and Diabetes, Hepcidin and Hemochromatosis
  5. Defining and Classifying Hemochromatosis
  6. Defining and Classifying Disorders due to Tissue Iron Accumulation
  7. Clinical Aspects
  8. Conclusions and Perspectives
  9. References